Offset lithography is a widely used printing process which utilizes a printing plate which has been treated so that certain portions of the plate are water receptive and other portions of the plate are receptive to an oil base ink. The printing process consists of first applying water to the sheet and then an oil base ink. After the alternate application of water and ink the aluminum sheet is then placed in contact with a rubber roll and a portion of the ink on the aluminum sheet is transferred to the rubber roll. The rubber roll is then placed in contact with a sheet of paper and the image which results on the paper is directly related to the original surface condition of the aluminum sheet. The aluminum sheet is usually prepared through the use of a photographic process. In one variation of this process, a photosensitive polymer is applied to the surface of the sheet and a pattern of light corresponding to the desired printed image is caused to impinge on the photosensitive polymer. Subsequently, a developer removes all the photosensitive polymer which was not exposed to light. Because of surface tension effects the oil base ink will adhere to the areas where the photosensitive polymer remains and the water will adhere to areas where the original surface of the aluminum sheet is exposed. Large numbers of copies may be made from one printing plate, sometimes in excess of one million. Because the resultant printed image depends on the surface condition of the aluminum sheet, it is highly important that the original surface of the sheet be smooth, flat and free from defects.
Aluminum alloys are widely used in the production of printing plates for use in offset lithography. Difficulties are encountered when aluminum alloy printing plates are used in extremely long production runs. These difficulties include fatigue cracking of the alloys and excessive wear of the alloy. These problems of low fatigue strength and excessive wear are both related to the inability of the alloy to further work harden in service. Commonly used aluminum alloys, Aluminum Association designation 3003 and 1100 have a fatigue strength in hard tempers of about 10,000 psi at 500,000,000 reversals.
These problems cannot be solved by the substitution of higher strength aluminum alloys because present commercial processes cannot produce material having the required width, flatness and surface finish in alloys having a tensile strength in excess of 35,000 psi.
The preceeding difficulties may be largely overcome through the use of the alloy of the present invention. The alloy of the present invention has a fatigue strength of between 13,000 and 15,000 psi and a tensile strength of about 25,000 psi. When used in a partially annealed condition, these strengths may be obtained while the alloy retains sufficient work hardening capabilities so as to minimize wear.